This invention relates to the use of ionic liquids in electroplating, and in particular for electroplating thick, hard chromium from trivalent salts.
Electroplating is an electrodeposition process for producing a thick, uniform, and adherent coating, commonly of metal or alloys, upon a surface by the act of electric current (see, M. Kulkarni et al, Bangladesh Journal of Scientific and Industrial Research, 2013, 48, 205-212). The coating formed changes the properties of the underlying substrate and is generally applied to improve wear and corrosion resistance of the interface or improve the aesthetic properties of the object. The piece to be electroplated is made into the negative electrode in an electrochemical cell and a current is passed through an electrolyte containing the ions of the metal to be electrodeposited.
There has been little change in the method of electroplating over 100 years and almost all processes are based on aqueous solutions of metal salts with a variety of additives to control morphology and properties. The industry is dominated by a relatively small number of coating materials. Anti-wear coatings are mostly Cr, Ni and Co and their alloys with other metals (M. Schlesinger and M. Paunovic, Modern Electroplating, John Wiley & Sons, 2010; and Z. Zeng and J. Zhang, Journal of Physics D: Applied Physics, 2008, 41, 185303).
The use of aqueous solutions has many issues for electroplating primarily due to the narrow potential window, and so metals with a large negative reduction potentials, e.g. Cr and Zn, are deposited with poor current efficiencies and suffer from hydrogen embrittlement (A. P. Abbott and K. J. McKenzie, Physical chemistry chemical physics: 2006, 8, 4265-4279).
Furthermore, although water is a green solvent, the inclusion of high metal concentrations means that the water has to be extensively cleaned before it can be returned to the environment (R. D. Rogers, K. R. Seddon, A. C. S. Meeting, Ionic Liquids As Green Solvents: Progress and Prospects, American Chemical Society, 2003). The electroplating process is also a complex series of pre- and post-treatment steps to prepare the substrate and remove the electrolyte after coating.
There are a number of key advantages of using aqueous solutions, such as:                Low cost        Non-flammable        High solubility of electrolytes        High conductivities resulting in low ohmic losses and good throwing power        High solubility of metal salts        High rates of mass transfer        
For these reasons, water will remain the backbone of the metal plating industry. Nevertheless, there are also limitations of aqueous solutions comprising:                Limited potential windows        Gas evolution processes can be technically not easy to handle and results in hydrogen embrittlement        Passivation of metals can cause issues with both anodic and cathodic materials        Requirement for complexing agents such as cyanide        All water must be returned to the water course        
These issues stop aqueous solutions being useful to the deposition of several technically vital materials. The main research areas in electroplating include replacement of environmentally toxic metal coatings (such as chromium), deposition of novel alloys and semiconductors and new coating methods for reactive metals.
Chromium plays an important role in a number of modern industries, for example, as a protective material in automotive and aerospace applications as well as for decorative purposes. It has almost unparalleled hardness and is used extensively for hydraulic systems. Chromium is traditionally electroplated from chromic acid which is a mixture of CrO3 and H2SO4. Although this has been the basis of a successful technology for over 50 years it is highly toxic and carcinogenic. There has been cumulative anxiety due to environmental, health and safety concerns related with the emission, treatment, storage which has led to reduced usage of hexavalent chromium compounds (K. Legg, M. Graham, P. Chang, F. Rastagar, A. Gonzales and B. Sartwell, Surface and Coatings Technology, 1996, 81, 99-105).
In general, hexavalent chromium electroplating baths produce trivalent chromium ions and hydrogen gas at the cathode, whereas oxygen gas is the major product at the anode. Hexavalent chromium is strongly linked with lung cancer and it also causes burns, ulceration of the skin and the mucous membrane, and loss of respiratory sensation.
In addition to its toxicity there are other issues associated with the deposition of chromium from chromic acid electrolytes. These have been summarized by Smart et al (Trans. Inst. Met. Finish., 1983, 61, 105-110) as follows:                Chromium electrodeposition utilising Cr(VI) has a low efficiency i.e. 15-22% where the remainder of the applied current is used in hydrogen evolution.        The average cathodic current densities are high (typically 10-15 Adm−2).        The procedure has poor covering power across low current density areas.        Burning is observed as grey deposits in high current density zones.        Chromium electroplating has low throwing power, which results in thick electrodeposits on the boundaries and protruding parts of cathodes and thin deposits over the rest of the surface.        Breaks in power during electrodeposition produces milky deposits known as white washing.        Chromic acid pose instant harmful effects on human tissue, burning the skin and even dilute solutions cause ulcers.        Chromic acid is a strong oxidizing agent and hence is a fire hazard.        High cost of chemical treatment.        
Numerous studies have attempted to develop trivalent chromium formulations for chromium plating and while several have been commercialised they are all used for decorative coatings. Trivalent chromium is at least 100 times less toxic to humans and the environment than hexavalent. Thermal spray techniques, nickel-based coatings and trivalent chromium electroplating have all been used as alternatives to Cr(VI) but none have comparable hardness.